1. Field of the Invention
Generally, a method for the regeneration of cationic exchange resins used for water softening is described. More specifically, by utilizing alkali (such as sodium and potassium) salts of .alpha.-hydroxy carboxylic or o-hydroxybenzoic acids, a method is presented for regenerating spent weak acid resins, such as alkaline earth polycarboxylates, back to their alkali salt form.
2. Description of the Background Art
Various ion exchange resins have been utilized in numerous procedures in industries, laboratories, general commercial facilities, and in private homes for many years to aid in softening hard water or generally in the removal of particular ions via ionic interactions. Hardness in household and industrial water is removed, in practice, by means of softening systems which replace the polyvalent ions such as Ca.sup.2+ and Mg.sup.2+ with sodium ion or other alkali metal ions. A commonly utilized process to accomplish this objective is to pass the water through an ion exchange column, wherein the polyvalent ions are exchanged for the sodium ions on the resin in the column. In time, the resin becomes saturated with the polyvalent ions and the column must be regenerated to put its resin back into the sodium form. Two general types of ion exchange resins are commonly used, the strong acid and weak acid resins. Polysulfonic acid resins are representative of the former and polyacrylic (or generally polycarboxylic) acid resins the latter.
The two types of ion exchange resins differ in three important respects which affect their applications. First they differ in their selectivity, i.e., the difference in affinity for sodium and for the polyvalent ions. Polyvalent hardness ions can be removed from brine solutions containing up to 5,000 ppm NaCl by using strong acid resins in the sodium form. However, with weak acid resins, hardness ions can be removed from brine solutions containing up to 50,000 ppm NaCl. High selectivity means more complete removal of the polyvalent ions. The second important difference between the two types of resins is the procedure for regeneration. Strong acid resins are usually regenerated with a large excess of brine, 3 to 5 times stoichiometry. The discharge of the resultant waste brine is environmentally undesirable and, in an increasing number of areas, discharge to a sewer is prohibited. Third, these strong acid resins do not satisfactorily remove the undesirable heavy metal ions such as Fe.sup.2+ and Cu.sup.2+ which are usually present. Hence, additional equipment is needed to accomplish this goal of Fe.sup.2+ and Cu.sup.2+ removal.
Although one step methods have been devised for certain limited situations, the regeneration of weak acid resins usually required a two step process. The existing one step processes have not proved to be as efficient as necessary for economic viability and, as a consequence, have not been widely used. Traditionally, the first step in the two step regeneration of weak acid resins involves the removal of the polyvalent ions with a dilute solution of a strong acid such as hydrochloric acid or its equivalent; this puts the resin in the hydrogen form. The second step is to put the resin into the sodium form for the water softening service cycle by treating the resin with a dilute solution of sodium hydroxide or sodium carbonate. Because two steps are entailed, cost is correspondingly higher than with one step methods, so that weak acid resins, in spite of their higher selectivity and capacity, are not usually employed in water softening.
In particular, disclosed in U.S. Pat. No. 4,753,736 is a method for a one step regeneration of a strong acid ion exchanger using citrate salts of volatile amines as the regenerant. These salts are utilized to remove undesirable cations from boiler water condensate. In general, the aqueous solutions of these regenerants comprise a volatile amine and an anion selected from anions which do not form precipitates with any cation present in the water that is treated. Because of the amines, however, this method is not suitable for the production of softened potable water.
A technical bulletin from Rohm and Haas (IE-209-74 of June 1977) describes a product termed "Amberlite DP-1." This product is a macroreticular polymethacrylic carboxylic acid cation exchange resin. Because of regeneration procedures used by Rohm and Haas, the waste load on the environment is minimized. The spent regenerant salt, soda lime, and sodium citrate are discarded directly to the sewer and can not be recycled economically.
Kunin in U.S. Pat. No. 4,083,782 discusses a one step procedure for regeneration of weak acid resins for conditioning acidic water containing hardness ions. Alkali salts of citric or fumaric acids, as well as sodium polyphosphate or sodium hexametaphosphate, were used as regenerants. The resin was claimed to be regenerated to 70-96% of its salt form. However, according to this patent, when using the citrate or fumarate regenerants, this means that the salt form of the resin is a mixture of sodium, calcium, and magnesium and only about 37.5% of the resin is regenerated to the sodium form which represents its actual softening capacity if a pH of less than 8.5 is to be achieved. The pH of the water from the service cycle using this process ranged from 7.0-9.5, depending upon the degree of regeneration.
Kunin provides in U.S. Pat. Nos. 4,071,446 and 4,116,860 a regenerant composition and method for regeneration of weak acid cation exchange resins that is suitable for use in residential buildings. The composition comprises alkali metal chloride, alkali metal carbonate, and a metal chelating polycarboxylate. The polycarboxylate has two or three acid groups. It is argued that the regeneration process yielded 70% regeneration but, as cited for the previous patent, the examples indicate that the regenerated resins have only about 37.5% of their capacity available for softening. The pH of the treated water ranged from 7.1 to 9.5, depending upon the degree if regeneration.
Described in U.S. Pat. No. 4,298,477 is a regeneration of zeolite based cation ion-exchange polishers. High-pressure boiler feed water is polished and softened with a cation ion-exchange resin, and the resin is regenerated with a readily ionizable salt of an amine-type corrosion inhibitor. This is not a suitable process for producing softened potable water since the amines would be present.
Even with the above referenced regeneration techniques, significant deficiencies endure with the existing procedures. The benefits of the subject regeneration procedure stand out over the currently available systems and include: 1) achieving nearly quantitative regeneration of the resin without using mineral acids, while recycling the regenerant and recovering the hardness minerals as granular solids; 2) generating no waste solution as embodied in the instant invention; 3) yielding softened water with a pH between 7.0 and 8.0 during the service cycle using a fully regenerated resin; and 4) utilizing a regenerant combination whose calcium and magnesium salt solutions remain precipitate or crystal free for more than a week at concentrations exceeding their individual solubilities up to a total concentration of about 2N.
Additionally and more specifically, the subject process has the following beneficial features:
1. Up to twice the volumetric capacity of conventional strong acid ion exchange resins. PA1 2. Nearly quantitative single step regeneration of the spent resins. PA1 3. Recycling of regenerants. PA1 4. Efficient removal of hardness ions and heavy metal ions such as troublesome Fe.sup.2+ from feed water streams. PA1 5. Removal of hardness ions and heavy metal ions in useful carbonate, hydroxide and/or oxide forms. PA1 6. Softened product water produced at a pH of approximately 7.5, using a fully regenerated polycarboxylic acid cation exchange resin.
The foregoing patents reflect the state of the art of which the applicant is aware and are tendered with the view toward discharging applicant's acknowledged duty of candor in disclosing information which may be pertinent in the examination of this application. It is respectfully submitted, however, that none of these patents teach or render obvious, singly or when considered in combination, applicant's claimed invention.